(a) Technical Field
The present invention relates to a composite separator for a polymer electrolyte membrane fuel cell (PEMFC). More particularly, the invention relates to a composite separator for a PEMFC and a method for manufacturing same. The inventive method involves bringing graphite foil layers into direct contact with each other when graphite foils are stacked on both sides of a carbon fiber reinforced composite material prepreg, thereby improving electrical conductivity in the thickness direction of the separator.
(b) Background Art
In general, a polymer electrolyte membrane fuel cell (PEMFC) is a device that generates electricity with heat and water by an electrochemical reaction between hydrogen and oxygen (or air) as reactant gases. The PEMFC has various advantages, such as, high energy efficiency, high current density, high power density, short start-up time, and rapid response to a load change as compared to the other types of fuel cells. For these reasons, it can be used in various applications, such as, a power source for zero-emission vehicles, an independent power plant, a portable power source, and a military power source, among other applications.
A conventional fuel cell stack is shown in FIG. 1 and described, as follows.
A conventional fuel cell stack includes a membrane-electrode assembly (MEA), which is positioned in the center of each unit cell of the fuel cell stack. The MEA comprises a solid polymer electrolyte membrane 60, through which hydrogen ions (protons) are transported, and an electrode/catalyst layer disposed on each of both sides of the polymer electrolyte membrane 60. The electrode/catalyst layer can include an air electrode (cathode) 61 and a fuel electrode (anode) 62, in which an electrochemical reaction between hydrogen and oxygen takes place.
Moreover, a gas diffusion layer (GDL) 40 and a gasket 41 are sequentially stacked on both sides of the MEA, where the cathode 61 and the anode 62 are located. A separator 30, including flow fields for supplying fuel and discharging water produced by the reaction, is located on the outside of the GDL 40. An end plate 50 for supporting the above-described components is connected to each of both ends thereof.
Therefore, at the anode 62 of the fuel cell stack, hydrogen is dissociated into hydrogen ions (protons, H+) and electrons (e−) by an oxidation reaction of hydrogen. The hydrogen ions and electrons are transmitted to the cathode 61 through the electrolyte membrane 60 and the separator 30, respectively. At the cathode 61, water is produced by the electrochemical reaction in which the hydrogen ions and electrons transmitted from the anode 62. The oxygen in the air participates and, at the same time, electrical energy is produced by the flow of electrons.
In the above-described fuel cell stack, the separator divides the unit cells of the fuel cell and, at the same time, serves as a current path between the unit cells. The flow fields formed in the separator serve as paths for supplying hydrogen and oxygen and discharging water produced by the reaction.
Since the water produced by the reaction inhibits the chemical reaction occurring in the electrolyte membrane of the fuel cell, the water should be rapidly discharged to the outside. Therefore, the separator material may have high surface energy such that the water is rapidly spread on the surface of the separator (hydrophilicity) or may have low surface energy such that the water rolls down the surface of the separator (hydrophobicity). Therefore, it is necessary to maximize the hydrophilicity or hydrophobicity of the flow fields in order to minimize the electrical contact resistance between the separators and facilitate the circulation of the produced water.
In consideration of these factors, conventional separators are formed of graphite, thin stainless steel, or a composite material in which expanded carbon particles or graphite particles are mixed with a polymer matrix.
By comparison, the electrical resistance of stainless steel is significantly lower than the electrical resistance of graphite.
However, since the electrical contact resistance is related to the contact area, pressure, and rigidity, the electrical contact resistance of graphite is lower than that of stainless steel. However, while the graphite satisfies the conditions of the separator in electrical and chemical aspects, it is vulnerable to impact and is difficult to process.
Therefore, a continuous carbon fiber composite separator which can improve the electrical, chemical, and mechanical properties would be an advance in the art. Moreover, a method for manufacturing a separator for a fuel cell which can reduce the contact resistance between unit cells, which is one of the most important electrical properties, and control the surface energy of a continuous carbon fiber composite separator to facilitate the discharge of water produced by the reaction has been required.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.